The present invention relates to a pretensioner for vehicle seat belts, and in particular to a pretensioner suitable for pretensioning a belt by pulling on a buckle to which the belt is connected.
In some seat belt systems used in vehicles such as automobiles, a pretensioner is provided for preventing movement of the body of an occupant by tightening of the belt when excessive acceleration is applied on the vehicle and thereby providing greater restraint of the occupant. Some pretensioners use a mechanical sensor having an inertia body for detecting acceleration to initiate the operation of a power source, such as a torsion bar, a compression coil spring, a spiral spring, an air cylinder, or the like, which is coupled to a component of the seat belt system and tightens the belt upon operation of the power source in response to a high acceleration detected by the mechanical sensor. Because the power source must produce a very large force in order to move the belt instantaneously before the occupant is thrown forward, the power source must be restrained by a very large force. Releasing the large restraining force requires that the inertia body be large and heavy.
Japanese Patent Laid-open Publication No. 1-119455 discloses a seat belt pretensioner in which the power source for applying pretension to the belt is an air cylinder, which is attached to a buckle support bracket, and the piston of the power source cylinder, which is moveable along the cylinder, serves as an inertia mass for initiating the operation of the air cylinder.
In order to provide it with sufficient mass to initiate operation of the pretensioner, the air cylinder piston must be large, which means that the cylinder also has to be large and heavy. The use of the air cylinder piston as the inertia mass is not an altogether satisfactory arrangement from the standpoint of making the pretensioner of light weight and compact construction.